The present disclosure relates to a battery charger capable of charging a battery pack of a secondary battery and outputting a stable direct-current power supply to an external power supply output terminal.
A battery charger for charging a secondary battery by using a commercial power supply has been known. In addition to the charging function, an use range of the battery charger may be widened if the electric power charged to the secondary battery can be used as an external direct-current (hereinafter referred to as “DC”) power supply. The inventor of the invention has proposed a battery charger disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-234139.
FIG. 7 shows a configuration of the battery charger disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-234139. A commercial alternating-current (hereinafter referred to as “AC”) power supply is converted to a DC power supply by an input filter 11 and a rectifier circuit 12. A switching power supply includes a pulse width modulation control circuit 13, a transistor Q11, and a transformer T1. The transistor Q11 which functions as a switching element switches, for example, at 100 kHz by an output pulse of the pulse width modulation control circuit 13. Rectified outputs from a diode D11 and a capacitor C11 which are connected to a tertiary winding N3 of the transformer T1 are supplied as a power source of the pulse width modulation control circuit 13.
Current flowing in a primary winding N1 is controlled by the transistor Q11, and electric power is induced in a secondary winding N2 and the tertiary winding N3. A voltage induced in the secondary winding N2 is rectified by a diode D12 and a capacitor C12, and the rectified output Vo is supplied to a charge control circuit 23 via output terminals 21 and 22. Also, the output is subject to resistance voltage division by a resistor R21 and a resistor R22, and thereafter inputted to a minus terminal of an operation amplifier AMP1. On the other hand, a reference voltage REF1 is inputted to a plus terminal of the operation amplifier AMP1, and an error signal detected by comparing the output voltage Vo and the reference voltage is connected to a photocoupler PH1 via a diode D13.
A voltage higher than the output voltage Vo is induced from a winding N22 of the transformer T1, and the induced voltage is rectified by a diode D15 and a capacitor C13 so that an output thereof is supplied to the photocoupler PH1 via a resistor. An error signal transmitted from a second side to a first side of the photocoupler PH1 is supplied to the pulse width modulation control circuit 13. The circuit 13 controls ON time period of an output pulse of the transistor Q11 and controls an electric power to be supplied to the second side such that an output voltage set by a reference voltage at the second side may be derived.
While an output current Io is supplied to a load circuit, the amount of current is detected by a resistor R12, and the detected current is inputted to a minus terminal of an operation amplifier AMP2. A reference voltage REF2 is supplied to a plus terminal of the operation amplifier AMP2, and a voltage of a plus terminal of the operation amplifier AMP2 is increased by an amount corresponding to that of the reference voltage. The flowing of output current Io causes a voltage drop of the resistor R12 due to the output current, and consequently, the reference voltage causes a voltage drop to a minus direction. Thus, when the amount of load current is increased, a voltage of a plus terminal of the operation amplifier AMP2 connected to the reference voltage REF2, goes down.
In this manner, a voltage of a plus terminal of the operation amplifier AMP2 goes down in accordance with the amount of currents flowing in the resistor R12 and the resultant voltage is compared with a voltage of a minus terminal. The operation amplifier AMP2 compares the amount of currents set by the reference voltage REF2 with the amount of currents flowing in the resistor R12, and an error signal detected by the comparison is inputted to the photocoupler PH1 via a diode D14. The error signal of the output current is provided to the pulse width modulation control circuit 13 as similar with the case of voltage control described above. Thus, the pulse width modulation control circuit 13 at the first side controls ON time period of the transistor Q11 such that the output current Io becomes a predetermined amount of current set by the reference voltage REF2.
In this manner, the operation amplifier AMP1 controls the output voltage Vo to be a predetermined voltage, and the operation amplifier AMP2 controls the output current Io to be a predetermined amount of current. The voltage outputted from the power supply device is supplied to the charge control circuit 23.
A configuration of the charge control circuit 23 is shown in FIG. 8. The charge control circuit is adapted to charge batteries BAT 21 and BAT 22 of nickel-metal-hydride secondary batteries. A plus terminal 21 of the power supply device is connected to the respective emitters of transistors Q21 and Q22, and the respective collectors of the transistors are connected to positive electrodes of the batteries BAT21 and BAT22. A minus terminal of the power supply device is connected to negative electrodes of batteries BAT21 and BAT22.
Outputs CH1 and CH2 of a controller 24 which includes a microcomputer, control switching of transistors Q21 and Q22 respectively so that charge currents are alternately supplied to the batteries BAT21 and BAT22, as shown in FIG. 9. In other words, when the transistor Q21 is in ON state and the transistor Q22 is in OFF state, charge currents flow to the battery BAT21, and when the transistor Q21 is in OFF state and the transistor Q22 is in ON state, charge currents flow to the battery BAT22.
Battery voltages of the batteries BAT21 and BAT22 are supplied to inputs AD1 and AD2 of an A/D converter of the controller 24, and the battery voltages converted to digital data are detected by the controller 24. Further, the controller 24 lights a charge display such as a LED 25 during charging. A voltage Vcc generated by the power supply device is supplied to the controller 24 as a power supply voltage via a regulated power supply circuit 27.
FIG. 10 shows typical variations of a voltage V and a current I of a nickel-metal-hydride secondary battery during charging. As the charging comes to the end, changes in a charge voltage peculiar to a nickel-metal-hydride secondary battery, such as an abrupt rise of voltage occurs and a voltage drops thereafter, occur. The voltage drop is typically express as −Δ, and the completion of charging is determined by detecting −ΔV which is voltage drop of a few mV.
A battery charger disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-234139 can charge a secondary battery, and also output a DC power supply for an external load. In this example, since a voltage of a secondary battery itself can be externally outputted, power supply can be provided in case of a power failure of the commercial power supply, but adversely overdischarge of a battery may be caused. Further, a nominal voltage of a nickel-metal-hydride battery is as low as around 1.2V, and therefore, even though a voltage is derived as an external power supply, the power supply may be unstable and its use range may be limited.
Japanese Unexamined Patent Application Publication No. 2004-304941, discloses a battery charger in which a battery is charged by an AC-DC conversion circuit and DC outputs are outputted externally. In other words, in the Japanese Unexamined Patent Application Publication No. 2004-304941, while an output from an AC-DC conversion circuit is externally outputted from a DC output circuit via a backflow prevention diode, an output from a battery is combined at the DC output circuit via a DC-DC conversion circuit.
Japanese Unexamined Patent Application Publication No. 2004-304941 discloses a configuration in which an output subjected to an AC-DC conversion charges a battery and is outputted externally. When a nickel-metal-hydride secondary battery or a lithium-ion secondary battery is charged, it is difficult to obtain a stable voltage required for an output of an AC-DC conversion circuit due to the change of the battery voltages.
Further, with respect to a nickel-metal-hydride secondary battery and a lithium-ion secondary battery, Japanese Unexamined Patent Application Publication No. 2004-304941 does not disclose specific numbers of battery cells constituting the batteries, and connection types of a plurality of batteries. For example, a battery voltage per a single cell of the nickel-metal-hydride secondary battery is as low as 1.0V to 1.4V, therefore, it is very difficult to stably operate a DC-DC converter at the battery voltage value, and unstable operation may be caused.
In the Japanese Unexamined Patent Application Publication No. 2004-304941, a secondary battery is connected to a DC-DC converter with no AC input, and therefore, a battery is consumed by constant operation of the DC-DC converter. Even if an output current from the DC-DC converter is zero, consumption of the battery may be caused due to consumption currents by DC-DC converter operations.
Therefore, it is desirable to provide a battery charger having an external DC power supply output, capable of outputting a stable DC power supply, capable of obtaining desired DC power supply outputs even if a secondary battery of a low battery voltage is used, and capable of reducing consumption currents of a battery.